An intraocular lens (“IOL”) can be used to replace a native lens of the eye when the native lens has been clouded by a cataract, or when the native lens loses part or all of its ability to accommodate. Non-accommodating IOLs have been described, including fixed monofocal IOLs and multifocal IOLs. Accommodating IOLs have also been described, which have accommodative capabilities similar to a native lens.
To replace a native lens with an IOL, the native lens is first removed from the capsular bag (typically by emulsification), leaving the capsular bag in the eye. The IOL is then implanted within the capsular bag. It is generally beneficial to ensure that the IOL to be implanted is appropriately sized based on the size of the patient's capsular bag. It may be even more important to ensure that an accommodating IOL is appropriately sized because, unlike a non-accommodating IOL, an accommodating IOL accommodates in response to changes in shape of the capsular bag. The accommodative response of an accommodating IOL may therefore depend on the appropriateness of the fit between the IOL and the capsular bag. Determining, or estimating, the size of the capsular bag before implanting the IOL is therefore generally beneficial, and may even greatly enhance the accommodative response of an accommodating IOL.
Techniques have been described to estimate the size of a capsular bag, but they have shortcomings which result in a need for improved methods of estimating the capsular bag size. For example, magnetic resonance imaging (MRI) can be used to non-invasively measure the dimensions of the capsular bag. The image resolution is, however, typically about ±0.1 mm or more. Moreover, the MRI slice thickness is generally too thick to get an accurate estimation of the true equatorial diameter of the lens capsule since there are typically only 3-5 images taken across the lens. Attempting to decrease the slice thickness creates a longer scanning time and this creates images with more motion noise as the patient's eye slightly moves over the course of the scan. Additionally, the access to, cost, and analysis of a MRI scan makes this technique prohibitive for IOL applications.
While optical coherence tomography (OCT) could be used to non-invasively measure the anterior lens radius, current clinical OCT devices do not have the capability to image a significant portion of the lens radii due to the iris. OCT measurements are currently made along or parallel to the optical axis of the eye. Therefore, the area of the lens surfaces that can be imaged is limited by the iris. Accurately calculating lens radii is highly dependent on the amount of lens surface (arc length) that can be imaged as well as ensuring axial alignment so that the true lens center is being imaged. As understood, OCT methods, unlike Scheimpflug methods, currently do not rotationally ‘scan’ the lens which is needed in order to reconstruct the true shape of the lens since asymmetries may be present. Like Scheimpflug imaging, OCT images also require distortion correction due to the different indices of refraction that the light travels through. OCT may a potential method if the issues mentioned above can be addressed.
Invasive methods such as capsular tension rings (see, e.g., Vass, C. et al. Prediction of pseudophakic capsular bag diameter based on biometric variables. J Cataract Refract Surg. 1999; 25:1376-1381, which is incorporated by reference herein) and capsule measurement rings (see, e.g., Tehrani, M. et al. Capsule measuring ring to predict capsular bag diameter and follow its course after foldable intraocular lens implantation, J Cataract Refract Surg. 2003; 29:2127-2134, which is incorporated by reference herein) have been used to estimate capsule size. These methods involve the implantation of a flexible, incomplete (<360°) ring which has an unrestrained diameter greater than the diameter of the capsular bag. When the ring is placed in the capsular bag after lens removal, the ring stretches out the capsule like a low-stiffness spring. By measuring the distance between features on the ring before and after implantation, a measurement of the stretched capsule diameter can be made. Since these methods are invasive and are deforming the capsular bag they can only obtain an equivalent diameter measurement and not a true volumetric measurement of the capsular bag, unlike Scheimpflug imaging (and possibly small-slice thickness MRI and rotationally scanning OCT). Additionally, invasive methods are not ideal as the correct size of the replacement IOL must be available at the time of measurement as opposed to non-invasive methods which allow the surgeon time to acquire the appropriate device or revise the treatment strategy.